Gourt Home::Gourt :: Heliospheric current sheet
The Heliospheric current sheet (HCS) is the surface within the Solar System where the polarity of the Sun's magnetic field changes from north to south. This field extends from the Sun's equatorial plane throughout the entire Solar System, and can be considered its largest structure.http://science.nasa.gov/headlines/y2003/22apr_currentsheet.htm The shape of the current sheet results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium (Solar Wind).http://quake.stanford.edu/~wso/gifs/HCS.html A small electrical current flows within the sheet, about 10-10 amps/m2. The thickness of the current sheet is about 10,000km.
The underlying magnetic field is called the interplanetary magnetic field, and the resulting electric current forms part of the heliospheric current circuit.Israelevich, P. L., et al, "MHD simulation of the three-dimensional structure of the heliospheric current sheet" (2001) Astronomy and Astrophysics, v.376, p.288-291 The Heliospheric current sheet is also sometimes called the Interplanetary Current Sheet.
Characteristics
Ballerina's skirt shape
As the Sun rotates, its magnetic field twists into a Parker spiral,Parker, E. N., "Dynamics of the Interplanetary Gas and Magnetic Fields", (1958) Astrophysical Journal, vol. 128, p.664 a form of an Archimedean spiral, named after its discovery by Eugene Parker. As the spiraling magnetic sheets changes polarity, it warps into a wavy spiral shape that has been likened to a ballerina's skirt.Rosenberg, R. L. and P. J. Coleman, Jr., Heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field, J. Geophys. Res., 74 (24), 5611-5622, 1969.Wilcox, J. M.; Scherrer, P. H.; Hoeksema, J. T., "The origin of the warped heliospheric current sheet" (1980) Further dynamics have suggested that "The Sun with the heliosheet is like a bashful ballerina who is repeatedly trying to push her excessively high flaring skirt downward".Mursula, K.; Hiltula, T., "Bashful ballerina: Southward shifted heliospheric current sheet]" (2003), Geophysical Research Letters, Volume 30, Issue 22, pp. SSC 2-1
Magnetic field
The heliospheric current sheet rotates along with the Sun once every 27 days, during which time the peaks and troughs of the skirt pass through the Earth's magnetosphere, interacting with it. Near the surface of the Sun, the magnetic field produced by the radial electric current in the sheet is of the order of 5x10-6T.
The magnetic field at the surface of the Sun is about 10-4 tesla. If the form of the field were a magnetic dipole, the strength would decrease with the cube of the distance, resulting in about 10-11 tesla at the Earth's orbit. The heliospheric current sheet results in higher order multipole components so that the actual magnetic field at the Earth due to the Sun is 100 times greater.
Electric current
The electric current in the heliospheric current sheet is directed radially inward, the circuit being closed by outward currents aligned with the Sun's magnetic field in the solar polar regions. The total current in the circuit is on the order of 3×109 amperes. As a comparison with other astrophysical electric currents, the Birkeland currents that supply the Earth's aurora are about a thousand times weaker at a million amperes. The maximum current density in the sheet is on the order of 10-10 A/m2 (10-4 amps/km2).
History
The heliospheric current sheet was discovered by John M. Wilcox and Norman F. Ness, who published their finding in a 1965 John M. Wilcox and Norman F. Ness, "Quasi-Stationary Corotating Structure in the Interplanetary Medium" (1965) Journal of Geophysical Research, 70, 5793.. Hannes Alfvén and Per Carlqvist speculateHannes Alfvén and Per Carlqvist, "Interstellar clouds and the formation of stars" (1978) in Astrophysics and Space Science, vol. 55, no. 2, May 1978, p. 487-509. on the existence of a galactic current sheet, a counterpart of the heliospheric current sheet, with an estimated galactic current of 1017 - 1019 Amps, that might flow in the plane of symmetry of the galaxy.
References
External links
"Heliospheric current sheet"